Membrane switches enjoy widespread use because they are inexpensive to manufacture, are easily custom configured, and are fairly reliable. Such uses include computer keyboards and electronic devices of almost every description. Common examples include such diverse products as video cameras, washing machines, hand held calculators and scientific and medical equipment. Even though membrane switches are in widespread use, two important inherent weaknesses are present in these switches.
A first weakness of membrane switches is that they have poor tactile feedback as provided to a human switch operator. One aspect of this poor tactile feedback relates to the initial travel of the switch. At the onset of switch motion, there is no distinctive tactile indication that the switch is starting to move. A tactile indication that a switch surface is just starting to move, but has definitely not begun to move far enough to reach its point of activation, is sometimes referred to as "tease". Membrane switch technology inherently has no, or very little, tease. Tease is considered desirable in many circumstances. For example, the shutter button on a camera requires or benefits from a tactile warning before its activation. This application is one in which the lack of tease of a membrane switch makes such a switch unsuitable; and other switch technologies generally are used. Even though membrane switches normally are not used to activate the shutter button on a camera, such membrane switches are used in less expensive and smaller or more easily configured switches to control less critical features on the camera. Mechanical switches, however, provide a tactile indication for the shutter activation that the switch is starting to move, and typically, a second indication that the switch is near its point of activation. This is not possible with conventional membrane switches.
A second lack of tactile feedback inherent in membrane switches is the poor ability of the operator to tactilely determine that the switch has been activated. Membrane switches which are used with a computer keyboard frequently employ a collapsible cylindrical collar made of silicone rubber to produce a breakaway sensation and a verification of activation, which is not otherwise provided by the membrane switch.
Another lack of tactile feedback of membrane switches relates to the ability of the operator to reduce the force necessary to hold the switch in its activated position, and thereby receive a warning that the operating key surface is starting to move toward the inactive position. To be most useful, this tactile warning sensation must be provided before the switch becomes inactive; so that an operator may calibrate the pressure applied to the switch by decreasing the pressure to the point where the tactile warning signal is felt. Once this warning is felt, the operator then may slightly increase the pressure just enough to move past this warning pressure zone. This is important because it allows a switch operator to calibrate the activation foreseen; so that excessive force is not required to guarantee that the switch is not inadvertently released.
A second primary inherent weakness in membrane switch technology is that membrane switches have little or no hysteresis. Consequently, these switches are known to undergo what is known as "chatter". Chatter is a situation in which the switch oscillates between its activated "on"position and its inactivated or "off" position. Hysteresis, as frequently used with switches, is a situation in which the "on" and the "off" positions are physically different positions. For example, a switch with hysteresis might, on its downward travel, need to be depressed three-fourths of its possible total travel in order to enter its "on" position. Such a switch, however, may not return to its "off" position until it has traveled past one-half the way up or in the return direction. The separation of one-fourth the travel between the "on" position and the "off" position prevents a situation in which tiny oscillations can cause the switch to chatter or oscillate between the "on" and "off" positions with tiny movements. A membrane switch typically turns on and off as contact is made or broken between two membranes, one of which is being deformed in the switch operation so as to contact the other. This situation provides little or no mechanical hysteresis.
The most common method used to improve the tactile operation of membrane switches is to combine such switches with separate hard key caps and plungers. Additionally, such key caps and plungers are generally provided with a method of producing tactile feedback. An important and common example of this approach is used in membrane based computer keyboard. Frequently, in such keyboard applications, a separate key cap and plunger are supported above the membrane switch by a silicon collar. The silicon collar is designed to collapse non-linearly as pressure is applied to it. Specifically, as the collar is depressed, its resistance to motion increases up to a point where the collar collapses in a buckling manner. When this occurs, the resistance to motion through the collar decreases. This increase and subsequent reduction of force frequently is referred to as tactile breakaway. The disadvantages of this system are increased complexity of the overall keyboard switch device, increased size (particularly with respect to the depth parallel to the direction of travel), and the fact that the silicon collars weaken and change resistance with heavy usage, over time.
An approach to improving tactile feedback of a membrane switch is disclosed in the United States patent to Goll, U.S. Pat. No. 3,681,723. This patent employs magnets to repel a pair of membrane sheets from one another; so that a pre-established force must be overcome in order to close the switch. All of the other disadvantages of conventional membrane switches, however, are present with this switch, since the magnets do nothing more than hold the sheets apart, essentially aiding in the resilient tendency of the sheets to hold the switch contacts open.
A different approach is disclosed in the U.S. patent to Michalski U.S. Pat. No. 4,349,712. This patent provides a tactile feedback through the use of a "dome" in the membrane material. This dome provides a "snap-over" or "snap" action operation. The membrane of the switch in Michalski is not a flat membrane; and the materials employed must be more rigid or spring-like than the membranes which are used in many other membrane switches. The switch of Michalski, however, does provide an indication when the switch moves from its open position to its closed position; although this is a single "snap-over" type of operation. Because of the nature of the switch, however, chattering also inherently is reduced.
It is desirable to provide an improved membrane switch having reduced chattering and enhanced tactile feedback, which is simple, easy to manufacture, of relatively low cost, and which can be used in conjunction with standard membrane switch materials.